Double check the authentication credentials to ensure that you are making the correct request.
If you have access to ServiceCenter, please activate the Trace for that REST API in order to check the payload you're sending and validate that it is correct.

<< check attached screenshots >>

After that, do another request and check service center integration logs to view the complete request payload.

are you using SetWebReferenceCredentials to set the Basic Authentication details first? I have not used rest in outsystems but soap for outsystems I have used SetWebReferenceCredentials but looking at Nuno screen shots rest outsystems implmentation looks easier.

That one does not work on REST, only SOAP from what I could see. It does not allow me to Add "SetWebReferenceURL" - I get the following error: The literal "webreference" in the argument WebReferenceName of the Execute Action SetReferenceURL does not match any Webservice name consumed in the eSpace

Did you use https://www.soapui.org/oauth2/reference/ntlm.html in your soapui project to get it to work in SOAPUI...are you willing to share the soapui project? Find it hard to investigate with out working example in soapui first.

Did you use https://www.soapui.org/oauth2/reference/ntlm.html in your soapui project to get it to work in SOAPUI...are you willing to share the soapui project? Find it hard to investigate with out working example in soapui first.

Yes I did, I used NTLM authentication in SOAPUI. Unfortunately since it is my company's specific WS, I can't provide that information.

If you are using this all on premise then I recommend using wireshark to sniff the packets as Nuno says.

What people should not be thinking is Outsystems is bad at REST...this is simply part of being a API developer, you need to be able to use Wireshark/Fiddler etc as when you come WS Security (aka Death Star) for examlple SOAPUI as a client is sometimes tooooooo good and does things behind the scences that you are then not aware of you have to replicate in your client tool (in this case Outsystems).

Outsystems staff can also do little to help like any other vendor until they have both headers to compare working SOAPUI and Outsystems created header.

If you use https://www.getpostman.com/ then you will get a better understanding of how to build the header before trying to get outsystems to build the header. You actually want to recreate the outsystems error in Soapui/Postman !!!